Strange things are happening in the North Sea. Cod stocks are slumping faster than over-fishing can account for, and Mediterranean species like red mullet are migrating north.

Several sea birds are also in trouble. Kittiwake numbers are falling fast and guillemots are struggling to breed.

And, earlier this summer, hundreds of fulmar (a relative of the albatross) corpses washed up on the Norfolk coast, having apparently starved to death.

Scientists suspect these events are linked and they are trying to work out how.

Nothing is certain yet, but some believe a dramatic change in North Sea plankton is responsible. And, what is more, they blame global warming.

Global changes

Plankton are microscopic free-floating marine organisms. Globally they are of vital importance.

Phyto-plankton (tiny plants) are behind 50% of the Earth's photosynthesis. And, along with zoo-plankton (tiny animals), they form the base of the whole ocean food web.

But, over the last 20 years, these little organisms have been undergoing a radical shake-up in the North Sea.

Everybody working in the North Sea is seeing big changes

Sarah Wanless, Nerc Centre for Ecology and Hydrology

Broadly speaking, as global temperatures rise, cold water species are moving out and warm water species are moving in.

"The North Sea was a cold temperate ecosystem in the 1980s, but since the 1990s it has changed into a warm temperate ecosystem," explained Martin Edwards, of the Sir Alistair Hardy Foundation for Ocean Science (SAHFOS), Plymouth, UK.

"So all the cold water species of plankton have moved much further north, and they are being replaced by more sub-tropical species."

Kittiwakes are declining rapidly

The decline of a particular species of phyto-plankton, which blooms in early spring, is key to changes further up the food chain.

Many small animals feed on this spring bloom - and even time their own emergence to match it. Now it is going, they are dying.

Dr Edwards said: "The spring bloom is declining, and the cold water zoo-plankton that feed on the spring bloom are declining as well - and so are the fish larvae that feed on the zoo-plankton."

And so - presumably - are other fish that feed on those larvae and the birds that feed on the fish.

Over the edge

Martin Edwards and bird expert Sarah Wanless, from Nerc Centre for Ecology and Hydrology, have got together to try to pinpoint how this is happening.

"We have already shown that kittiwakes are declining and declining quite rapidly - but at the moment we don't know the mechanisms," said Dr Wanless.

"And that is why we are joining up the bird work with the plankton work, so we can see how these links actually operate.

Amazingly the role of plankton has not been extensively studied in the past

"What Martin Edwards and his colleagues are showing is that there is a profound difference in the plankton in the North Sea, and we are seeing a decline in the sand eels that many birds feed on."

She continued: "We speculate these are climate driven changes, which are working their way right through the food chain. And we are seeing signals emerging from the birds.

"In some cases we are finding a whole lot of adult birds dead and in other cases that the birds are abandoning their chicks."

And there might be worse to come. Because sea birds are generally long lived, changes happen rather slowly. So what we are seeing now could be, some fear, the tip of the iceberg.

"Some species might decline to the extent where they are no longer present," said Dr Wanless. "And kittiwakes might well be one of the first to disappear."

Emerging picture

Amazingly the role of plankton, and how they are affected by climate, has not been extensively studied in the past.

"Work on plankton is hugely important," said Dr Chris Reid, the Director of SAHFOS.

"The world's oceans make up more than 70% of the world's surface and 50% of the primary production in the world comes from phyto-plankton, but as yet we have very little idea about how this changes regionally or with time."

Now, the story behind some of the North Sea mysteries is beginning to unfold.

Dr Wanless said: "Everybody working in the North Sea is seeing big changes, but what we need to do now is get everybody together, to try to piece the whole jigsaw together."